1. Field of the Invention
The present invention relates to a surface treatment aqueous solution for providing a conversion coating on a metal surface such as iron or iron alloy, especially on the surface of tinned iron. This invention also relates to a use of the surface treatment aqueous solution and to a process of applying the surface treatment aqueous solution to the surface of tinned iron.
2. Description of the Prior Art
Iron cans are broadly used for various canned products, as a container for food or soft drinks. Such cans are fabricated by shaping a thin iron plate into a desired size of can using a forming die or a drawing & ironing technique (DI technique). In either method, a shaped can is generally provided with an anticorrosion coating because the material itself is inferior in corrosion resistance. As an anticorrosion coating, a thin film of epoxy resin is ordinarily formed on the can's surface. Resin coating is not directly provided on the iron surface, but on the treated surface. Such a surface treatment includes a conversion coating which is preferable in view of corrosion resistance and coating adhesiveness especially for tinned iron cans.
Japan Kokai (Laid Open) H2-15178, entitled "Conversion Coating Agent and Conversion Coating Bath", discloses a conversion coating on a tinned can, using an aqueous solution containing orthophosphoric acid and/or orthophosphate with a PO.sub.4 amount 1 to 30 g/l, oxalic acid and/or oxalate with a C.sub.2 O.sub.4 amount 0.005 to 5.0 g/l, and 0.005 to 0.5 g/l of dihydric tin ion, with a pH value 3 to 5. The oxalic acid is added to a treatment bath as an etchant and as a chelating agent for tin ions for preventing sludge.
U.S. Pat. No. 4,927,472 entitled "Conversion Coating Solution For Treating Metal Surface" discloses an aqueous solution containing 1 to 50 g/l of phosphate ions, 0.2 to 20.0 g/l of oxyacid ions, 0.01 to 5.0 g/l of tin ions, and 0.01 to 5.0 g/l of condensed phosphate ions, with a pH value 2 to 6. The condensed phosphate ions are added as an etchant as well as a chelating agent for tin ions for preventing sludge.
Japan Kokai (SHO)52-53739 entitled "Surface Treating Method for Tin Plate" discloses a method for treating a surface of a tinned steel plate, using an aqueous solution in which one or both of phosphorous acid and hypophosphorous acid is/are added to phosphoric acid and the amount of free acid is adjusted to 5.0 to 50.0 g/l. The phosphorous acid and hypophosphorous acid are added as an etchant or a coating accelerator.
U.S. Pat. No. 4,980,076 entitled "Fluoride and Chromium Free Acid Etchant Rinse For Aluminum" discloses an acid etchant rinse for aluminum containing orthophosphoric acid in an amount to give a stoichiometric equivalent of 3.0 to 50 g/l as PO.sub.4.sup.3-, an aluminum ion sequestrant component in an amount of 0.01 to 10.0 g/l, 20 to 170 ppm of ferric ion, and 0.02 to 3.0 g/l of H.sub.2 O.sub.2 or NO.sub.2.sup.-. As an aluminum ion sequestrant, organophosphonate is employed.
However, the conversion coating agent shown in H2-15178 has a problem that the oxalic acid contained in the treatment bath is slightly inferior in chelation ability to tin ions, and therefore it can not sufficiently hold tin ions. As a result, sludge is produced in the treatment bath, which will block a nozzle.
It is known that condensed phosphate is easily hydrolyzed in a strong acidic aqueous solution to be a phosphorate with a poor degree of polymerization. For this reason, the conversion coating solution disclosed in U.S. Pat. No. 4,927,472 has a problem that the condensed phosphate contained in the solution is quickly hydrolyzed at pH value 2 to 5 suitable for conversion coating, and chelation ability to tin ion is extremely reduced, which results in precipitate of tin salt and production of sludge. It is not economical to continuously supply condensed phosphate ions as hydrolysis advances. Further, the oxyacid promotes elution of iron ions which consume the chelating agent, resulting in insufficient trapping of tin ions and production of sludge.
In Japan Kokai (SHO)52-53739, inorganic hypophosphorous acid, such as sodium dihydrogenphosphate or ammonium dihydrogenphophate, is added as an etchant or coating accelerator. However, inorganic hypophosphorous acid is inferior in chelation ability, and therefore, accumulation of tin ions produces sludge.
The solution shown in U.S. Pat. No. 4,980,076 is an acid etchant rinse for treating aluminum or aluminum alloy, where ferric ion and an oxidizer are indispensable. This solution differs from s surface treatment aqueous solution of the present invention in structure, purpose and effect.